1. Technical Field
Exemplary aspects of the present disclosure generally relate to an electrophotographic image forming apparatus, more particularly to a transfer bias control employed in an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as a copier, a printer, a facsimile machine, and a digital multi-functional system including a combination thereof generally employ an electrophotographic method.
The image forming apparatuses of this kind form an electrostatic latent image by charging uniformly a surface of a photosensitive member and illuminating the charged surface with laser light associated with an image data. The thus-obtained electrostatic latent image is developed with toner to form a toner image. The toner image on the surface of the photosensitive member is transferred directly onto a recording medium or is transferred onto an intermediate transfer member before transferring secondarily the toner image onto the recording medium. The toner image transferred onto the recording medium is fixed by a fixing device. After fixing, the recording medium is discharged outside the apparatus.
In such an image forming apparatus, for example, in a copying machine, there is a time lag between a time at which users press a copy-start button and a time at which a first sheet of recording medium on which an image is formed is discharged. This time lag is also known as a first copy time (FCOT) during which the users have to wait, and shortening this first copy time leads to improving operating efficiency of the users.
In the image forming apparatus, other than the copying machine, the time lag also occurs when carrying out a new print job. More specifically, a time period until the first image is output after printing is instructed corresponds to the first copy time mentioned above. In this case, this time period is referred to as a first image output time (FCOT).
The first image output time (FCOT) can be shortened by accelerating a process linear velocity. However, in order to reduce power consumption, in general, it is necessary to reduce the process linear velocity, thereby complicating efforts to shorten the first image output time as is usually desired. In other words, efforts to shorten the first image output time contradict efforts to reduce the power consumption.
Reducing a preparation time for printing in an image forming unit can shorten the first image output time (FCOT), which does not affect energy saving characteristics. Therefore, shortening the preparation time for printing in the image forming unit can lead to reduction of the waiting time for the users, and hence there is demand for a short preparation time for printing.
In view of the above, various approaches have been proposed in an attempt to adjust the time at which a transfer bias (a primary transfer bias in the case of an intermediate transfer method using the intermediate transfer member) is applied to a transfer member such as a transfer roller to transfer the toner image from the photosensitive member to a recording medium or to the intermediate transfer member.
In order to facilitate an understanding of the novel features of the present disclosure, as a comparison, a description is provided of a charging position, a writing position, a developing position, and a transfer position in a related-art image forming apparatus using the electrophotographic method with reference to FIG. 10. In FIG. 10, the related-art image forming apparatus includes a photosensitive drum 102 that rotates in the direction of arrow A, a charging roller 200 serving as a charger, a developing roller 300 serving as a developer bearing member of a developing device, and a transfer roller 400 serving as a transfer device.
A place on a surface 102a of the photosensitive drum 102 charged by the charging roller 200 is referred to as a charging position B. A place on the surface 102a at which an electrostatic latent image is formed due to exposure with laser light L from a writing unit is referred to as a writing position C. A place on the surface 102a at which the electrostatic latent image is developed by the developing roller 300, hence forming a toner image, is referred to as a developing position D. A place on the surface 102a at which the transfer roller 400 transfers the toner image onto a recording medium or an intermediate transfer belt is referred to as a transfer position E.
A time at which a charging bias (voltage) starts to be supplied to the charging roller 200 is referred to as a charging bias output timing. A time at which a transfer bias (voltage) starts to be applied to the transfer roller 400 is referred to as a transfer bias output timing.
More specifically, the transfer bias output timing is a time at which the transfer bias is output when the charging position B of the surface 102a of the photosensitive drum 102 arrives at the transfer position E after the charging bias output timing.
The surface 102a of the photosensitive drum 102 is normally charged to a negative potential. At the transfer position, the polarity of the surface 102a shifts to a positive polarity due to the transfer bias. If, after the first charging of the surface 102a, the surface 102a of the photosensitive drum 102 includes a charged portion to which the transfer bias is not applied and a portion which has not been charged and hence the transfer bias is applied thereto, a trace of the transfer bias remains easily at the electrical potential on the surface 102a of the photosensitive drum 102 when charging for the second time. As a result, when forming a halftone image or the like, an electrical potential difference generated on the first sheet of image formation causes a difference in the image density of a developed image, which appears as a horizontal streak perpendicular to a direction of conveyance of the recording medium in an output image.
To address this difficulty, in one approach, after a position (hereinafter referred to as charging start position) on the surface 102a of the photosensitive drum 102 from which charging of the surface 102a is started for the first time passes the transfer position E, the transfer bias starts to be supplied. The place to which the transfer bias starts to be applied is charged for the second time at the charging position B and arrives at the writing position C. Subsequently, writing is started. With this configuration, the above difficulty is prevented.
Although advantageous, there is a relatively long time lag between the start of charging and the start of image formation, resulting in a relatively long first image output time (FCOT).
The image forming apparatus may include a charge removing device upstream from the charging position B in the direction of rotation of the photosensitive drum 102 so as to remove residual charge on the surface 102a of the photosensitive drum 102 prior to the subsequent charging, that is, the second charging and thereafter. With this configuration, the trace of the transfer bias does not remain, and writing can be started when the leading end of the charged portion of the surface 102a arrives at the writing position C. However, providing the charge removing device to remove the charge on the surface 102a over the entire axial direction of the photosensitive drum 102 is expensive.
In view of the above, the transfer bias is supplied when the charging start position of the surface 102a arrives at the transfer position as described above.
In this configuration, when the charging start position of the surface 102a arrives at the transfer position E, the transfer bias is applied thereto and the surface potential of the surface 102a shifts to the positive side. Thereafter, the charging bias is applied continuously, and hence the electrical potential difference after the second charging is reduced.
Accordingly, even when writing is started at a time at which the first charging start position of the surface 102a arrives at the writing position, the horizontal streak due to the difference in image density hardly appears. Furthermore, since image formation can be started immediately after the start of charging, the first image output time (FCOT) can be shortened.
Supplying the transfer bias when the charging start position of the surface 102a arrives at the transfer position is advantageous in that the difference in the image density can be reduced. However, a thin horizontal streak may still appear. This is because when the timing is met perfectly, overshoot is generated at the rise timing of the output of the transfer bias and the trace of the transfer bias remains strongly at a local area equivalent of several tens of milliseconds (ms), much stronger than other areas, which appears as a thin black streak in a resulting output image.
The degree of the overshoot depends on the rise time of the surface potential of the photosensitive drum 102 facing the transfer roller 400. It is generally the case that the rise time takes approximately one hundred milliseconds when the charging bias is applied by a known charging roller, for example, the charging roller 200, regardless of a contact-type or contact-free charger.
When applying the transfer bias to the surface 102a of the photosensitive drum 102 having a surface potential with the aforementioned rise time, a response from a power source of the transfer bias is delayed regardless of constant current control or constant voltage control. Consequently, overshoot of ten to twenty percent greater than a predetermined electrical current value or a voltage value occurs in the transfer bias for several tens of milliseconds upon rising and gets stabilized thereafter.
The surface potential of the portion of the photosensitive drum 102 corresponding to the area in which the overshoot has occurred shifts to the positive side relative to the area, the surface potential of which is stabilized. As a result, the trace of the transfer bias remains even after the subsequent charging or the second charging. When starting the image formation early so that these areas arrive at the image forming region fast in order to shorten the first image output time, the trace of overshoot in the transfer bias appears as a thin black streak in a halftone image.
In view of the above, JP-2010-26083-A proposes a primary transfer bias subjected to constant current control when transferring a toner image from the photosensitive member onto the intermediate transfer belt in the intermediate transfer method.
In this approach, based on an observed surface potential of the photosensitive member, a target electrical current value for the transfer bias is determined. For example, for the photosensitive member having a relatively high surface potential, a relatively high primary transfer bias is applied. Accordingly, when the surface potential of the photosensitive member is set such that the difference between the potential at the first charging and the potential at the second charging is relatively large, the potential difference is smoothed by a higher transfer bias. Thus, the density difference is difficult to occur.
However, the image forming apparatus proposed in JP-2010-26083-A does not specify the timing at which the primary transfer bias is supplied. In a case in which the image formation is initiated so as to shorten the first image output time (FCOT), a horizontal black streak as mentioned above is generated. Furthermore, JP-2010-26083-A is silent with respect to generation of overshoot.
In another approach, according to JP-4072532-B1 (JP-2006-145605-A), a band of toner is adhered to the photosensitive member in advance by calculating backwards based on the output timing of the transfer bias in the direct transfer method so as to prevent the toner from getting transferred to a recording medium. When the transfer bias is supplied, the band of toner lies in the transfer nip, thereby preventing the trace of transfer bias from remaining due to overshoot.
In JP-4072532-B1 (JP-2006-145605-A), an area with the intervening toner band and an area without the intervening toner band are created so that irregular image density occurs when forming an image. Although this configuration prevents the trace of transfer bias due to overshoot, the history of output of the primary transfer bias, that is, whether the primary transfer bias has been output, appears as a difference in the image density.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of producing an image without irregular image density and an undesirable horizontal streak while shortening the first image output time (FCOT).